1. Field of the Invention
The present invention relates, in general, to an apparatus and method for automatically correcting a bias voltage for a carrier suppressed pulse generating modulator using the phase distribution of the output pulse, which performs automatic control to automatically detect an optimal bias voltage for the carrier suppressed pulse generating modulator used in a carrier suppressed return-to-zero modulator and to maintain the optimal bias voltage during the operation of the modulator.
2. Description of the Related Art
Generally, in a long distance optical transmission system using Wavelength Division Multiplexing (WDM), the modulation of a transmission signal has been performed by a Non-Return-to-Zero (NRZ) modulation method using a Mach-Zehnder type external modulator. An external modulator used in this case exhibits a phenomenon (a DC bias drift) in which a transfer curve moves laterally due to the variations in a temperature, etc. Accordingly, an optical transmission signal is distorted due to the DC bias drift, so that an extinction ratio is deteriorated and unstable power is output, thus deteriorating the performance of the system. Therefore, the technology of automatically correcting a bias voltage is required so as to output a stable signal regardless of the temperature variations.
Currently, various technologies related to a method or an apparatus for correcting bias voltages for modulators based on NRZ modulation have been proposed.
However, recently, as a transmission rate of an optical transmission network increases and an interval between channels narrows, various modulation schemes based on Return-to-Zero (RZ) modulation instead of conventional NRZ modulation have been researched.
A Carrier Suppressed Return-to-Zero (CSRZ) modulation (hereinafter referred to as “CSRZ modulation”) scheme shown in FIG. 1 is one of optical modulation schemes, which have been newly researched.
Referring to FIG. 1, the CSRZ modulator includes a Mach-Zehnder-type first external modulator (Mach-Zehnder Interferometer 1: MZI1) 101 for receiving a laser beam from a laser light source to generate carrier suppressed pulses, and a second external modulator (Mach-Zehnder Interferometer 2: MZI2) 102 with a single port for modulating the carrier suppressed pulses output from the first external modulator 101 using NRZ modulation.
A bias voltage of the first external modulator 101 is located at the bottom point of its transfer curve as indicated by reference numeral S201 in a graph of FIG. 2, so that the laser beam is modulated at a voltage amplitude of 2Vπ in synchronization with signal clock/2. As a result of the modulation, the laser beam is converted into a pulse signal indicated by reference numeral S202 of FIG. 2, and a phase difference of 180 degrees exists between neighboring pulses. At this time, referring to the spectrum of the pulse signal, a carrier is suppressed and is not shown.
The pulse signal S202 is applied to the second external modulator 102. A bias voltage of the second external modulator 102 is located at the point indicated by reference numeral S203 of FIG. 2, that is, the mid-point of the slope of its transfer curve, so that the second external modulator 102 performs an ON/OFF function of gating a signal on and off in response to a data signal, that is, a NRZ modulating function. As a result of the NRZ modulation, a pulse signal carrying data is generated as indicated by reference numeral S204.
There is no problem even though the second external modulator 102 employs a conventional bias control scheme without change; however, the first external modulator 101 for generating the carrier suppressed pulse cannot employ a bias voltage stabilizing method, which was used in the conventional NRZ modulation, without change in accordance with the operating characteristics thereof. Therefore, there is required a new scheme capable of performing stable bias control with respect to the first external modulator 101 employing the CSRZ modulation.